Adaptive zero current sense apparatus and method for a switching regulator

ABSTRACT

A switching regulator includes a low-side switch having a body diode. During the low-side switch is on, a zero-current sense circuit monitors the inductor current of the switching regulator and triggers a signal to turn off the low-side switch when the inductor current falls down to a zero-current threshold, to prevent reverse inductor current from the output terminal of the switching regulator. A body-diode turn-on time controller monitors the turn-on time of the body diode and adjusts the zero-current threshold according thereto, and the turn-on time of the body diode can be reduced to an optimal interval subsequently. The self-adjustable zero-current threshold is adaptive according to the application conditions, such as the inductor size, input voltage and output voltage of the switching regulator.

FIELD OF THE INVENTION

The present invention is related generally to a switching regulator and,more particularly, to a zero-current sense apparatus and method for aswitching regulator.

BACKGROUND OF THE INVENTION

According to the inductor current condition, switching regulators can begenerally categorized into two types: Continuous Conduction Mode (CCM)and Discontinuous Conduction Mode (DCM). FIGS. 1 and 2 are waveformdiagrams of the ideal inductor currents in CCM and DCM switchingregulators, respectively, and both the modes have respective advantagesand applications. FIG. 3 shows a conventional DCM synchronous buckswitching regulator 100, in which a controller 101 switches a high-sideswitch 110 and a low-side switch 112 connected in series between a powerinput terminal Vin and a ground terminal GND to generate an inductorcurrent IL that charges a capacitor Co to produce an output voltageVout, and voltage divider resistors R1 and R2 are connected in seriesbetween the output terminal Vout and the ground terminal GND to generatea feedback voltage VFB for the controller 101 to regulate the outputvoltage Vout. In the controller 101, an error amplifier and compensatorunit 102 generates an error signal VEA according to the feedback voltageVFB, a Pulse Width Modulation (PWM) comparator 104 generates a PWMsignal according to the error signal VEA, a logic and driver unit 106switches the switches 110 and 112 in response to the PWM signal toconvert the input voltage Vin to the output voltage Vout, and azero-current sense circuit 108 senses the inductor current IL by sensingthe voltage on the phase node LX, and triggers a zero-current detectionsignal ZCDET for the logic and driver unit 106 to turn off the switch112 when the inductor current IL falls down to a zero-current thresholdIth during the switch 112 is on, in order to prevent reverse inductorcurrent IL from the output terminal Vout.

FIG. 4 is a waveform diagram showing the inductor current IL and thevoltage on the phase node LX of the switching regulator 100, in whichwaveform 200 represents the inductor current IL, and waveform 202represents the voltage on the phase node LX. At time t1, the high-sideswitch 110 is turned on and the low-side switch 112 is turned off, andthereby the inductor current IL begins to increase with the slopeslope_rise=(Vin−Vout)/L.  [Eq-1]At time t2, the high-side switch 110 is turned off and the low-sideswitch 112 is turned on, and thereby the inductor current IL begins todecrease with the slopeslope_fall=Vout/L.  [Eq-2]When the inductor current IL falls down to the zero-current thresholdIth, as shown at time t3, the low-side switch 112 is turned off toprevent reverse inductor current IL from the output terminal Vout. Asshown by the waveform 202 in FIG. 4, between times t3 and t4, a bodydiode 114 of the switch 112 is turned on to remain the inductor currentIL flowing from the ground terminal GND to the output terminal Voutthrough the phase node LX, and until the inductor current IL becomeszero the body diode 114 turns off.

However, the zero-current threshold Ith of the conventional zero-currentsense circuit 108 is fixed but not adjustable when it is designed, andin the event that the output voltage Vout or the inductor L is changed,causing the falling slope slope_fall of the inductor current IL changed,the pre-set zero-current threshold Ith becomes not suitable to the realconditions. For example, when the falling slope slope_fall of theinductor current IL is changed to be much steeper as shown by thewaveform 204 in FIG. 5, the low-side switch 112 may not be turned off intime after the inductor current IL falls down to the zero-currentthreshold Ith, the voltage on the phase node LX may become positiveeventually, as shown by the waveform 206 in FIG. 5, and a reverseinductor current IL occurs. On the contrary, when the falling slopeslope_fall of the inductor current IL is changed to be much gentler asshown by the waveform 208 in FIG. 6, the body diode 114 is eventuallyconductive for a longer time period after the inductor current IL fallsdown to the zero-current threshold Ith, as shown by the waveform 210 inFIG. 6, and more wasted power consumption occurs.

Therefore, it is desired a zero-current sense apparatus and method withan adjustable zero-current threshold for a switching regulator.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide azero-current sense apparatus and method with an adjustable zero-currentthreshold for a switching regulator.

According to the present invention, a zero-current sense apparatus andmethod for a switching regulator monitor the inductor current of theswitching regulator, to trigger a first signal when the inductor currentfalls down to a zero-current threshold during the low-side switch is on,to turn off the low-side switch of the switching regulator, and monitorthe body diode turn-on time of the low-side switch after the low-sideswitch is turned off in response to the first signal, to determine asecond signal to remain or adjust the zero-current threshold. If thebody diode turn-on time is detected within a given value, thezero-current threshold is locked at the current level, otherwise thezero-current threshold is reduced. With the adjustable zero-currentthreshold, the switching regulator may turn off the low-side switchbefore the voltage on the phase node goes high across a pre-setzero-point, and adjust the timing within a given time interval.

Preferably, the zero-current threshold is self-adjusted according to theslope of the inductor current, which is dependent on the input voltage,output voltage and inductor size of the switching regulator. Therefore,the proposed apparatus and method can be adaptive according to theapplication conditions.

Advantageously, the proposed apparatus and method can prevent reverseinductor current from the output terminal of a switching regulator, andalso control the body-diode turn-on time of the low-side switch within agiven value.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of the presentinvention will become apparent to those skilled in the art uponconsideration of the following description of the preferred embodimentsof the present invention taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a waveform diagram of the ideal inductor current in CCMswitching regulators;

FIG. 2 is a waveform diagram of the ideal inductor current in DCMswitching regulators;

FIG. 3 shows a conventional DCM synchronous buck switching regulator;

FIG. 4 is a waveform diagram showing the inductor current and thevoltage on the phase node of the switching regulator of FIG. 3;

FIG. 5 is a waveform diagram showing the inductor current and thevoltage on the phase node of the switching regulator of FIG. 3 when theinductor current has an excessively steep falling slope;

FIG. 6 is a waveform diagram showing the inductor current and thevoltage on the phase node of the switching regulator of FIG. 3 when theinductor current has an excessively gentle falling slope;

FIG. 7 shows an embodiment for zero-current sense apparatus for aswitching regulator according to the present invention;

FIG. 8 is a flowchart of zero-current sense method for a switchingregulator according to the present invention;

FIG. 9 is a waveform diagram showing the signals in the circuit of FIG.7;

FIG. 10 is a first embodiment for the zero-current sense circuit in FIG.7;

FIG. 11 is an embodiment for the comparator in FIG. 10;

FIG. 12 is a second embodiment for the zero-current sense circuit inFIG. 7; and

FIG. 13 is an embodiment for the body-diode turn-on time controller inFIG. 7.

DETAIL DESCRIPTION OF THE INVENTION

As shown in FIG. 7, in a switching regulator 300, a controller 301switches a high-side switch 312 and a low-side switch 314 connected inseries between a power input terminal Vin and a ground terminal GND, togenerate an inductor current IL that charges a capacitor Co to producean output voltage Vout, the low-side switch 314 has a body diode 316,and voltage divider resistors R1 and R2 are connected in series betweenthe output terminal Vout and the ground terminal GND, to generate afeedback voltage VFB for the controller 301 to regulate the outputvoltage Vout. In the controller 301, an error amplifier and compensatorunit 302 generates an error signal VEA according to the feedback voltageVFB, a PWM comparator 304 generates a PWM signal according to the errorsignal VEA to provide for a logic and driver unit 306 to drive theswitches 312 and 314, a zero-current sense circuit 308 monitors theinductor current IL by monitoring the voltage on the phase node LX andtriggers a zero-current detection signal ZCDET when the inductor currentIL falls down to a zero-current threshold Ith during the switch 314 ison for the logic and driver unit 306, and a body-diode turn-on timecontroller 310 provides a signal S according to the voltage on the phasenode LX and the zero-current detection signal ZCDET for the zero-currentsense circuit 308, to remain or adjust the zero-current threshold Ith,thereby causing the turn-on time of the body diode 316 remained oradjusted. The zero-current threshold Ith, and thereby the turn-on timeof the body diode 316, can be self-adjustable by the internal circuit ofthe controller 301 according to the slope of the inductor current IL,which is dependent on the input voltage Vin, output voltage Vout andinductor size of the switching regulator 300.

FIG. 8 is a flowchart of zero-current sense method according to thepresent invention for the switching regulator 300 of FIG. 7. FIG. 9 is awaveform diagram showing the signals in the switching regulator 300 ofFIG. 7, in which waveform 500 represents the inductor current IL,waveform 502 represents the voltage on the phase node LX, waveform 504represents the zero-current detection signal ZCDET, and waveform 506represents a signal LX_RISING_DET which indicates the rising edge of thevoltage on the phase node LX. With reference to FIGS. 7-9, in Step 400,the high-side switch 312 is turned off and the low-side switch 314 isturned on at time t1. Assuming that the initial zero-current thresholdIth of the zero-current sense circuit 308 is Ith_1, Step 402 will checkwhether or not the inductor current IL is smaller than the zero-currentthreshold Ith_1. If it is not the case, the process returns to the Step400; otherwise, namely the inductor current IL is smaller than thezero-current threshold Ith_1, as shown at time t2, the process entersStep 404, in which the zero-current detection signal ZCDET transits tohigh to turn off the low-side switch 314 as shown by the waveform 504 inFIG. 9, and the body diode 316 turns on to remain the inductor currentIL flowing in the current direction. Then, Step 406 monitors the voltageon the phase node LX and checks whether or not it is across azero-point, which indicates the reference voltage and is 0V in thisembodiment. This step may be performed by using a zero-point detector todetect the voltage on the phase node LX. If the voltage on the phasenode LX is not greater than zero, the process returns to the Step 404.Contrarily, if the voltage on the phase node LX is greater than zero,the process proceeds to Step 408, in which the zero-current detectionsignal ZCDET transits to low as shown at time t3 in FIG. 9, and the bodydiode 316 is consequently turned off. Then in Step 410, the body-diodeturn-on time controller 310 checks whether or not the turn-on time Td ofthe body diode 316, namely the time period that the zero-currentdetection signal ZCDET stays at high level, is longer than a pre-setvalue Topt. At this time the body-diode turn-on time is Td_1, as shownbetween times t2 and t3 in FIG. 9. If it is detected the currentbody-diode turn-on time Td_1 is longer than Topt, then Step 414 willdecrease the zero-current threshold Ith from Ith_1 to Ith_2, and theSteps 400-410 will repeat with the new zero-current threshold Ith_2.Subsequently, the next turn-on time Td of the body diode 316 will beshorter, as shown by Td_2 between times t4 and t5 in FIG. 9. If in Step410 the turn-on time Td of the body diode 316, i.e. Td_2 at this time,is still longer than Topt, the zero-current threshold Ith will befurther reduced from Ith_2 to Ith_3, and then the Steps 400-410 repeatagain with the new zero-current threshold Ith_3. Once the turn-on timeTd of the body diode 316 eventually becomes equal to or shorter thanTopt, such as that between times t6 and t7 in FIG. 9, Step 412 will lockthe current zero-current threshold Ith, for example Ith_3 at this time.Through this process, the zero-current sense circuit 308 isself-calibrated by the body-diode turn-on time Td and the driver delaytime. The parameter Topt is preferably an optimal body-diode turn-ontime or a target for the turn-on time Td of the body diode 316, whichmay bring the switching regulator 300 into best efficiency operations,especially at light load condition.

FIG. 10 provides an embodiment for the zero-current sense circuit 308 ofFIG. 7, in which a comparator 602 has two input terminals 604 and 606connected to the phase node LX and the ground terminal GND,respectively, and an output terminal to trigger the zero-currentdetection signal ZCDET. When the inductor current IL flowing through thephase node LX during the low-side switch 314 is on falls down to becomelower than the zero-current threshold Ith, the voltage on the phase nodeLX will go high across zero, and therefore the output ZCDET of thecomparator 602 transits to high. The output ZCDET of the comparator 602will not transit back to low unless the inductor current IL becomeszero. In this embodiment, the signal S from the body-diode turn-on timecontroller 310 includes signals Reduce and Latch, and an N-bit counter600 will change or remain the setting of the comparator 602 according tothe signals Reduce and Latch, so as to adjust or remain the zero-currentthreshold Ith. FIG. 11 is an embodiment for the comparator 602, in whicha bipolar transistor 616 has an emitter serving as the input terminal604, and another bipolar transistor 618 has a common base with thebipolar transistor 616 and an emitter serving as the input terminal 606.As shown in FIGS. 7-9, when the voltage on the phase node LX is−Ith×Ron, where Ron is the on-resistance of the low-side switch 314, thezero-current detection signal ZCDET transits to high. Then, according tothe characteristics of the bipolar transistor, it determines thezero-current thresholdIth=VT[ln(1+1+2+ . . . m)]/Ron,  [Eq-3]where VT is the thermal voltage and m is a power of 2. From the equationEq-3, it is shown that the zero-current threshold Ith is determined bythe collector current Ic of the bipolar transistor 616. The N-bitcounter 600 controls switches 608 to 614 according to the signals Reduceand Latch from the body-diode turn-on time controller 310 to determinethe collector current Ic and thereby adjust the zero-current thresholdIth.

FIG. 12 provides a second embodiment for the zero-current sense circuit308 of FIG. 7, in which an N-bit counter 700 controls a voltage source704 to provide an offset voltage Voffset according to the signals Reduceand Latch, and a comparator 702 has an input terminal 706 connected tothe phase node LX with the voltage source 704 therebetween, and anotherinput terminal 708 grounded. When the voltage on the phase node LX risesup such that the inputs 706 and 708 of the comparator 702 are equal, thezero-current detection signal ZCDET is triggered and transits to high.In further detail, when the voltage on the phase node LX is −Ith×Ron,the zero-current detection signal ZCDET transits to high to turn off thelow-side switch 314, and the body diode 316 is turned on to remain theinductor current IL flowing in the current direction. The zero-currentdetection signal ZCDET will not transit back to low unless the voltageon the phase node LX becomes greater than zero. The N-bit counter 700remains or adjusts the zero-current threshold Ith by controlling thevoltage source 704, and the body-diode turn-on time controller 310determines the signals Reduce and Latch according to the turn-on time ofthe body diode 316 for the N-bit counter 700, so as to control theoffset voltage Voffset and thereby remain or adjust the zero-currentthreshold Ith. Alternatively, the voltage source 704 may be connectedbetween the input terminal 708 of the comparator 702 and the groundterminal GND with inverse polarity, to adjust the zero-current thresholdIth.

FIG. 13 is an embodiment for the body-diode turn-on time controller 310of FIG. 7. The zero-current detection signal ZCDET is connected to aninput terminal of a NOR gate 806 through an inverter, a zero-pointdetector 804 monitors the voltage on the phase node LX to determine asignal LX-RISING-DET to send into another input terminal of the NOR gate806, and the output of the NOR gate 806 is used as the input D of aflip-flop 800. The optimal body-diode turn-on time Topt is preset andconnected to the clock input terminal CK of the flip-flop 800 through aninverter 802, and an enable signal EN is connected to the reset inputterminal R of the flip-flop 800 to turn on or off the flip-flop 800.Referring to FIGS. 9 and 13, when the inductor current IL falls down tothe threshold Ith at time t2, the zero-current detection signal ZCDETtransits to high, the body diode 316 is turned on, and the voltage onthe phase node LX becomes lower than zero. At this time, the outputLX-RISING-DET of the zero-point detector 804 is still low as shown bythe waveform 506, and the NOR gate 806 triggers a high-level signal D tothe flip-flop 800. Until the voltage on the phase node LX rises up tozero at time t3, the body diode 316 is turned off, and the signalLX-RISING-DET transits to high, such that the signal D transits to low.If the time duration that the signal D stays at high level is longerthan the optimal body-diode turn-on time Topt, the output Reduce of theD flip-flop 800 will be high and reduce the zero-current threshold Ithof the zero-current sense circuit 308, for example by the N-bit counter600 or 700. On the contrary, if the time duration that the signal Dstays at high level is shorter than or equal to the optimal body-diodeturn-on time Topt, the signal Latch will be high and remain the currentzero-current threshold Ith of the zero-current sense circuit 308.

As illustrated by the above embodiments, the proposed apparatus andmethod can prevent reverse inductor current from the output terminal ofa switching regulator, and also control the body-diode turn-on time ofthe low-side switch within a given value. More particularly, theproposed apparatus and method have self-calibration mechanism to adjustthe zero-current threshold, and can control the zero-current thresholdto self-adjustable level, which is dependent on the application circuitfor any inductor size and input/output voltages. Furthermore, theproposed apparatus and method can reduce the body-diode turn-on time toan optimal interval, and thereby improve the light-load efficiency.

While the present invention has been described in conjunction withpreferred embodiments thereof, it is evident that many alternatives,modifications and variations will be apparent to those skilled in theart. Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and scopethereof as set forth in the appended claims.

1. An adaptive zero-current sense apparatus for a switching regulatorincluding a low-side switch connected to a phase node and switched tocontrol an inductor current, the low-side switch having a body diode,the apparatus comprising: a zero-current sense circuit for monitoringthe inductor current during the low-side switch is on, to trigger afirst signal to turn off the low-side switch if the inductor currentfalls down to a zero-current threshold; and a body-diode turn-on timecontroller for monitoring a turn-on time of the body diode, in order todetermine a second signal according thereto, to adjust the zero-currentthreshold.
 2. The adaptive zero-current sense apparatus of claim 1,wherein the zero-current sense circuit comprises: a comparator fortriggering the first signal by monitoring a voltage on the phase node,and turning off the first signal thereafter when the voltage on thephase node rises up to be higher than a reference voltage; and a counterfor adjusting the zero-current threshold according to the second signal.3. The adaptive zero-current sense apparatus of claim 2, wherein thecomparator comprises: a first bipolar transistor having an emitterconnected to the phase node and a collector for receiving an adjustablecurrent to determine the zero-current threshold; a second bipolartransistor having a common base with the first bipolar transistor and anemitter connected with the reference voltage; a plurality of currentsources; and a plurality of switches, each connected between one of theplurality of current sources and the collector of the first bipolartransistor and switched by the counter, for determining the adjustablecurrent.
 4. The adaptive zero-current sense apparatus of claim 2,wherein the zero-current sense circuit further comprises a voltagesource connected between the phase node and an input terminal of thecomparator for providing an offset voltage controlled by the counter todetermine the zero-current threshold.
 5. The adaptive zero-current senseapparatus of claim 2, wherein the body-diode turn-on time controllercomprises: a zero-point detector for triggering a third signal when thevoltage on the phase node rises up across the reference voltage; a logiccircuit for triggering a fourth signal according to the first signal andthe third signal; and a D flip-flop for triggering the second signalaccording to the fourth signal, to reduce the turn-on time of the bodydiode when the turn-on time of the body diode is longer than a givenvalue.